At six weeks of pregnancy, the embryo measures roughly 5 to 9 millimeters from top to bottom, about the size of a pomegranate seed or a small lentil. Technically, it’s still classified as an embryo at this stage, not a fetus. That transition doesn’t happen until about eight weeks after fertilization, according to the American College of Obstetricians and Gynecologists. Still, this tiny structure is changing rapidly, and a lot is already taking shape.
Overall Size and Shape
The measurement used at this stage is called crown-rump length, which is simply the distance from the top of the head to the bottom of the torso (there are no measurable legs yet). At exactly six weeks, the 50th percentile measurement is about 5 millimeters. By six weeks and six days, it’s closer to 9 millimeters. That range is normal and depends on the exact day within the week and natural variation between pregnancies.
The overall shape is curved, sometimes described as a C-shape or comma. The head end is noticeably larger than the rest of the body because the brain is one of the first organs to begin developing. There’s a visible bulge where the head is forming, and a tapering tail-like structure at the opposite end that will eventually disappear as development continues.
Facial Features Just Starting to Form
At six weeks, there’s no recognizable face. What exists instead are the earliest building blocks of one. Small dark spots mark where the eyes will eventually develop, with shallow grooves forming above and below each eye spot. These grooves are the very beginnings of eyelid folds, though actual eyelids won’t take shape for another couple of weeks.
The components of the upper lip are starting to come together, with two separate tissue prominences on either side of where the nose will be. Tiny pits mark the locations of future ear canals. The overall appearance of the head at this stage is more like a collection of folds and thickenings than anything resembling a human face. Features like a defined nose, visible ears, and a rounded head don’t appear until around week eight.
Limb Buds, Not Limbs
Arms and legs at six weeks are small paddle-shaped bumps called limb buds. Upper limb buds appear slightly before lower ones, so the arm buds are a bit more developed at this point. Nerves are already growing into the upper limb buds, and early muscle cells are lining up along the length of each bud. But there are no fingers, toes, or joints yet. The limb buds look like tiny rounded projections sticking out from the sides of the body, roughly proportional to grains of rice on the already tiny embryo.
A Heartbeat You Can Detect
One of the most significant developments at six weeks is a beating heart. The heart is a simple tube-like structure at this stage, not yet divided into four chambers, but it’s already contracting rhythmically. The typical heart rate when first detectable on ultrasound is about 100 to 120 beats per minute. That’s slower than it will be in a few weeks (it typically peaks around 170 bpm near week nine before settling down), but it’s fast enough to be visible as a flickering motion on a transvaginal ultrasound.
A heartbeat is generally detectable once the embryo reaches about 5 to 7 millimeters in length, which lines up with the six-week mark for most pregnancies.
Internal Organs in Early Stages
Inside the embryo, several organs are in the earliest phases of development. The brain is growing quickly and already has distinct regions beginning to form, which is part of why the head appears disproportionately large. The liver is developing and will soon begin producing blood cells, a job it handles before the bone marrow takes over later in pregnancy. The musculoskeletal system, the framework of bones and muscles, is also being laid down.
The lungs and kidneys are in their most primitive stages at this point. These organs go through multiple rounds of development over the coming weeks and won’t be functional until much later in pregnancy. What exists at six weeks are essentially the starter tissues that will eventually become those organs.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at six weeks, you won’t see anything that looks like a baby. What’s visible is a dark circular area called the gestational sac, which is the fluid-filled space where the embryo is developing. Inside that sac, there are two small structures: a round yolk sac (which provides nutrients to the embryo at this stage) and the fetal pole, which is the embryo itself.
The fetal pole appears as a small, bright, slightly elongated shape next to the yolk sac. On most ultrasound screens, it’s only a few millimeters long and can be difficult to distinguish without the technician pointing it out. If the timing is right, you may also see a tiny flicker within the fetal pole, which is the heartbeat. A standard abdominal ultrasound often can’t pick up these details at six weeks because the embryo is so small, which is why transvaginal ultrasounds are used this early.
It’s also worth knowing that not seeing a heartbeat at exactly six weeks doesn’t necessarily mean something is wrong. Even a difference of two or three days in gestational age can mean the difference between a visible heartbeat and one that’s just a bit too early to detect. In many cases, a follow-up ultrasound a week later will show clear cardiac activity.
Embryo vs. Fetus: Why the Terms Matter
Many people search for “6-week fetus,” but biologically, the correct term at this stage is embryo. The embryonic period covers the first eight weeks after fertilization, and this is when all the major organ systems are being established for the first time. It’s a period of rapid, foundational development. After eight weeks, the organism is reclassified as a fetus, which marks a shift from building new organ systems to growing and refining the ones already in place.
The distinction isn’t just academic. The embryonic period is when the developing organism is most sensitive to disruptions, which is why early pregnancy guidelines focus heavily on avoiding certain medications, alcohol, and environmental exposures during these first weeks.